1. Field of the Invention
The present invention relates to an image-forming apparatus and to a fixing device for use in the image-forming apparatus.
2. Description of the Related Art
On-demand fixing devices of which a start-up time is short are well-known. An on-demand fixing device includes a fixing film (endless film) as a fixing member, a pressure roller (pressure member), and a heater (heating unit) such as a ceramic heater. The fixing film is sandwiched between the heater and the pressure roller, and the fixing film is heated with the heater. When a recording medium with a toner image thereon passes through a nip section between the heater and the pressure roller, the recording medium is subjected to heat and pressure at the nip section, whereby the toner image is fixed to the recording medium. Such a technology has been disclosed, for example, in Japanese Patent Publication Laid-open No. 2002-6656.
On the other hand, Japanese Patent Publication Laid-open No. 2003-337484 discloses technology where heating bodies are provided on either surface of a heater board at a film-heating heating apparatus. One heating body is shorter than the other heating body. A current is supplied to only the shorter heating body when a small-sized paper passes. This prevents increases in temperature at the paper-feed portions.
Further, Japanese Patent Publication Laid-open No. 2001-356621 and Japanese Patent Publication Laid-open No. 2000-206826 disclose a technology where, in a film-heating heating apparatus, a second temperature detection element for sensing increase in temperature at sections where paper does not pass is housed within a housing at sections where the paper does not pass. This prevents a heating body or heating member etc. from degrading or breaking down as a result of increases in temperature at sections outside of where the paper passes when a recording material of the minimum width passes.
With the conventional on-demand fixing device such as that disclosed in Japanese Patent Publication Laid-open No. 2002-6656, the ease with which a heating unit such as a heater can be changed (ease of maintenance) is poor. This is explained in detail below.
The lifespan of the heater is finite, and maintenance to replace the heater is performed regularly. However, the heater is in contact with the pressure roller via the fixing film. This makes it difficult to pull out the heater in a lateral direction (longitudinal direction) as is in a state where pressure is applied.
In order to resolve this situation, one approach could be to provide a mechanism for releasing pressure between the heater (the fixing film) and the pressure roller before pulling out the heater. In other words, the heater can be pulled out from the apparatus after releasing the pressure between the heater and the pressure roller by manipulating the mechanism for releasing pressure. Providing a pressure release mechanism in such situations does, however, incur extra costs and require more space.
In particular, pressure is always applied to the heater by the pressure member in the conventional on-demand fixing device. This means that it is easy for the heater to be damaged when there is a paper jam or during transportation.
Further, when a recording medium of a width smaller than the maximum size for which paper is passed through is fed through (hereinafter “small size paper”) with the on-demand method fixing device of Japanese Patent Publication Laid-open No. 2003-337484, Japanese Patent Publication Laid-open No. 2001-356621, and Japanese Patent Publication Laid-open No. 2000-206826, heat is not dissipated by the recording medium at parts that the paper is not fed through. Sections that the paper is not fed through therefore reach high temperatures compared with sections that the paper is fed through. When increases in temperature at sections where paper is not fed through worsen, a heater, a heater stay, a fixing film, and a pressure roller are also damaged. This can also result in the degradation of the image quality. In particular, when a thick recording medium (referred to as small size thick paper such as cardboard and envelopes) smaller in width than the maximum size is transported and fed, large quantity of heat is dissipated to the recording medium at paper-feed portions. Temperature control is then performed based on the output of temperature detection elements provided at the paper-feed portions. The quantity of electrical power supplied to the heater is therefore substantial. On the other hand, heat is not dissipated by recording material at sections where paper is not fed through. The temperature therefore becomes high and there is the possibility of damage to the heater, the heater stay, the fixing film, and the pressure roller.